The titanium dioxide pigment is hydrophilic, so that its affinity toward organic resins is low and its dispersibility and filling ability into the plastic system are low. Particularly, in case of thin film processing, lacing (foaming) or pinholes are liable to occur due to water contained in the titanium dioxide pigment. In this connection, it is known to coat the surface of titanium dioxide pigment with an organic silicone compound to give an affinity toward plastic resins and a hydrophobic property both thereto.
Among the organic silicone compounds, hydrolyzable alkylsilane compounds are widely used in the plastic resin field. This is because the hydroxyl groups of hydrolysis products react with the hydroxyl groups present on the surface of titanium dioxide pigment to form chemical bonding, thereby effectively improving the hydrophobic property of titanium dioxide pigment and the affinity toward plastic resins. Further, the alkyl groups of the hydrolysis products on the other hand are inert toward the organic compounds, and have distinguished selectivity to plastic resin species.
However, generally the hydrolysis products of hydrolyzable alkylsilane compounds with a distinguished hydrophobic property have a low heat resistance, and the titanium dioxide pigment coated therewith has such problems that the pigment powder turns yellowish by heating in the drying and pulverizing steps and furthermore the plastic moldings also turn yellowish.